In what can only be described as a wholly unsurprising turn of events, I'm an idiot. I had not, in fact, installed GCC yet in UnixWare, only a package *vaguely related* to GCC. Turns out UnixWare's developers have a really nice single command to install a whole dizzying array of devtools - patchck -o -dev - including GCC, which is now doing its thing.
I'm so impressed by UnixWare. More people need to be able to explore this.